Embodiments of the present invention relate to fossil fuel power plant emissions and, more particularly, to particulate traps for filtering particulates from the emissions of a fossil fuel power plant.
Fossil fuel power plants are energy conversion centers that combust fossil fuels to produce electricity. A fossil fuel power plant converts the chemical energy stored in fossil fuels such as coal, fuel oil, or natural gas into thermal energy, then mechanical energy, and finally electrical energy for distribution and use. A modern and efficient fossil fuel power plant is capable of cleanly and efficiently converting a large percentage of the chemical energy stored in fossil fuels into electrical energy.
The majority of fossil fuel power plants in the U.S. rely on the combustion of coal. It is an object of the energy conversion process to convert as much chemical energy from coal into electrical energy. Achieving minimal loss in the energy conversion process is crucial to the success of the power plant for many reasons, especially in light of new and more restrictive environmental regulations and the rising costs of fossil fuels.
Clean and efficient power production for coal fired power plants depends in large part on the ability of the system to combust a high percentage of the coal input. The efficiency of coal combustion is measured in terms of Loss on Ignition (“LOI”). LOI refers to the percentage of unburned carbon output from the combustion process. Unburned carbon equates to unburned, wasted fuel, and thus increased operating costs. Therefore, it is a goal of a coal fired power plant to keep the LOI percentage as close to zero as possible. It will be understood by those of skill in the art that a LOI of zero is a theoretical limit, as there will always be some loss due to a number of factors, including furnace design, type of fuel, and operating conditions. For most power plant systems, plant operators strive to achieve a LOI of less than ten percent.
In addition to the desire to minimize LOI, it is important to be able to control the emissions of the coal combustion process, such control is necessary to limit the emission of pollutants, and to meet environmental guidelines and requirements. The coal fired combustion process creates numerous by-products, or off-products, in addition to the primary product of heat. The combustion process produces certain gas and solid emissions. These emissions are primarily contained in a fly ash output of the combustion process. The solid matter emissions are typically solid particulate by-products of coal combustion, which are entrained in the fly ash. The solid particulate is comprised of both inorganic components and organic components. Exemplary inorganic components include silicon dioxide (SiO2), aluminum oxide (Al2O3) and iron oxide (Fe2O3). Organic components are primarily carbon derivatives. These carbon derivatives most often take the form of soot and char. Soot and char are unburned fuel residues composed mainly of amorphous carbon.
The conventional means for filtering particulate from the emissions of the coal combustion process is to use electrostatic precipitators. An electrostatic precipitator is a particulate collection device capable of removing particles from flowing gas using the force of an induced electrostatic charge. An electrostatic precipitator has a negative voltage energy field and a positive voltage energy field. In practice, the flowing gas passes first through the negative voltage energy field, thus negatively charging. Then the flowing gas passes through the positive voltage energy field, and thus the negatively charged solid particulate matter is attracted to, and collected on, a positively charged collecting plate. To be effective, the solid particulate must be capable of accepting the negative charge and maintaining that charge for a sufficient time while migrating from the negatively charged field to the positively charged field.
In many instances, the solid particulate has an electrical resistivity between the range of approximately 10×105 to 10×1011 ohm-centimeters (ohm-cm) to effectively collect and dissipate the charge. Particulate with too low a resistive value may charge very quickly, but then discharge as soon as it exits the negative voltage energy field. Thus, such particulate may not be attracted to the positively charged collection plate by the time the particulate migrates to an area proximate the plate. Particulate with too high a resistive value may charge very slowly or essentially not charge, and thus not be attracted to the collection plate when flowing in the area proximate the plate. Thus, if the fly ash contains particulate of an improper electrical resistivity, then a large percentage of that particulate may not be filtered from the fly ash by the electrostatic precipitator and may potentially be permitted to exit the system as undesirable emissions.
Conventional electrostatic precipitators are effective and efficient at filtering the particulate that exhibits an electrical resistivity in the appropriate range, such as the inorganic components of the fly ash particulate. Conventional electrostatic precipitators, however, are relatively ineffective in filtering the organic components of the fly ash particulate. The organic components are comprised primarily of carbon, which is an electrical conductor, and thus, the organic components do not exhibit the necessary electrical resistivity to be effectively filtered by the electrostatic precipitator.
The problem of unfiltered particulate is exacerbated when the LOI levels of a combustion system increase. The conductive carbon can lower the resistivity levels of combined particulate matter, and thus the percentage of unfiltered particulates in general can increase. Increased particulate emissions can be detrimental to the power plant and can possibly exceed the allowable emissions limits. Power plants that exceed regulatory emission levels can be subject to fines, restrictions, and other detrimental measures.
For many reasons, it is highly undesired to emit organic particulate containing carbon from the stacks of the coal fired power plant. Not only are certain levels of carbon emission pollutants in violation of environmental regulations, they are also highly visible emissions. Carbon particulate emissions have a high opacity and, therefore, create an objectionable stack appearance. Carbon particulate emissions are primarily dark particles and are high in surface area, making these particles more visible in the atmosphere. Thus, the opacity of carbon emissions can detrimentally affect both the environment and the public's perception of the power plant. In addition to meeting environmental regulations, power plant operators desire to remain in good standing with their surrounding community, and thus strive to limit opaque emissions from plant stacks.
The ineffectiveness of the conventional filtration methods in removing organic particulate is increasingly problematic in light of recent changes to the operation of many coal fired power plants. The Clean Air Act Revision of 1990, 42 U.S.C. §7401 et seq., provides tight restrictions on nitrous oxide emissions. In order to meet these restrictions, many power plants have implemented modifications to their processes to delay the emission point of the combustion system and extend the burnout period of the ignited fuel in the furnace. A goal of these modifications is to extract a maximum same amount of thermal energy from the coal fuel, but to do so at lower flame temperatures to minimize the creation of excess nitrogen oxide. These modifications have been successful at lowering the nitrous oxide emissions of many plants, but, at the same time, these modifications have resulted in an increase in LOI. More specifically, the changes to reduce nitrous oxide have increased the levels of unburned carbon fuel released from the combustion system. As organic particulate is often primarily comprised of carbon, it is thus increasingly important to enable efficient and effective filtration of organic particulate.
In the late 1990s and early 2000s, certain power plants were experiencing high LOI as a result of compliance technology for nitrous oxide reduction, which technology is often referred to as Lonox. Lonox burners resulted in high carbon production. More recently, the industry seeks to reduce mercury emissions. Just as carbon is effective at absorbing radical metals such as mercury, carbon is also effective at reducing mercury emissions. Therefore, carbon is now being introduced into the system to reduce mercury emissions. When carbon is trapped by the filtration system, other particles, such as mercury, absorbed by or otherwise associated with the carbon, can be trapped and desirably filtered along with the carbon. Accordingly, the need to reduce carbon emissions becomes even more imperative.